Zeolite ZSM-5, described in U.S. Pat. No. 3,702,886 to Argauer et al., is known to be a shape-selective zeolite having an intermediate pore size, i.e. a pore size smaller than Zeolite X or Y but larger than Zeolite A. Since its discovery, ZSM-5 and related zeolites, more fully defined below, has been shown to be capable of efficiently catalyzing some unusual hydrocarbon (and other) conversions, and a large number of patents have issued describing such conversions. For example, U.S. Pat. No. 4,120,910 to Chu describes the conversion of ethane to aromatic hydrocarbons by contact at about 1100.degree. F. At much lower temperatures, such as at about 650.degree. F., it is known the higher linear and singly methyl-branched alkanes having six or more carbon atoms may be reacted with benzene, for example, to form alkylbenzenes and lower alkanes, presumably by reaction of olefins formed in situ from the alkane (by acid cracking) with the benzene.
It is generally recognized by those skilled in the art that alkanes having eight or more carbon atoms are relatively easily cracked to lower molecular weight hydrocarbons when contacted with an acidic catalyst, and that the reaction proceeds with very little disporportionation to paraffins of higher molecular weight than the charge. It is also generally known that as the molecular weight of the charged paraffin decreases, the reactivity of the paraffin also decreases.